The present invention relates to the field of combustion technology. It concerns a combustion apparatus, in particular for driving gas turbines, in which combustion apparatus a gaseous fuel in a burner is sprayed through a plurality of separate fuel-injection devices into a gas flow containing combustion air, and the resulting mixture flows into a combustion chamber for combustion where the mixture is burned.
U.S. Pat. No. 4,932,861, for example, discloses such a combustion apparatus, which is based in particular on a double-cone burner.
Thernoacoustic combustion instabilities can seriously impair safe and reliable operation of modern gas turbines with premixing. One of the mechanisms responsible for these instabilities is based on a feedback loop which includes the pressure and velocity fluctuations during the fuel-injection, the (convective) fuel in homogeneity transported by the flow, and the heat-release rate.
A fundamental stability criterion for the occurrence of thermnoacoustic combustion instabilities is the Rayleigh criterion, which can be formulated as follows:
As soon as a flame is enclosed in an acoustic resonator, thermoacoustic self-excited vibrations may occur if                                           ∫            0            τ                    ⁢                                    Q              xe2x80x2                        ⁢                          p              xe2x80x2                        ⁢                          ⅆ              t                                       greater than         0                            (        1        )            
where Qxe2x80x2 is the instantaneous deviation of the integral heat-release rate from its average (steady) value, pxe2x80x2 designates the pressure fluctuations, and T designates the period of the vibrations (1/T=f is the frequency of the vibrations). In the formula (1), it is assumed that the spatial extent of the heat-release zone is sufficiently small in order to work with integral values of Qxe2x80x2 and pxe2x80x2. Extension to the general situation with a distributed heat-release rate Qxe2x80x2 (x) and a small acoustic wavelength is obtained directly and leads to a so-called Rayleigh index. The Rayleigh criterion (1) states that, an instability can only occur if fluctuations in the heat release and in the pressure are at least in phase up to a certain degree.
In a combustion apparatus with premixing, the instantaneous heat-release rate depends, inter alia, on the instantaneous fuel concentration in the premixed fuel/air mixture which enters the combustion zone. The fuel concentration in turn may be influenced by (acoustic) pressure and velocity fluctuations in the vicinity of the fuel-injection device, provided that the air feed and the fuel-injection device are not acoustically stiff. This last-mentioned condition is normally fulfilled, i.e. the pressure drop of the air flow along the fuel-injection region of the burner is normally quite small, and even the pressure drop along the fuel-injection device is generally not large enough in order to uncouple the fuel-feed line from the acoustics in the combustion apparatus. The relationship between the acoustics at the fuel-injection device and the heat release in the flow can be formulated with the simplest expressions as follows:                                                         Q              xe2x80x2                        ⁡                          (              t              )                                Q                =                                                            u                xe2x80x2                            ⁡                              (                                                      x                    1                                    ,                                      t                    -                    τ                                                  )                                                    u              ⁡                              (                                  x                  1                                )                                              -                                    1              2                        ⁢                                                            p                  xe2x80x2                                ⁡                                  (                                                            x                      1                                        ,                                          t                      -                      r                                                        )                                                            Δ                ⁢                                  xe2x80x83                                ⁢                p                                                                        (        2        )            
where x, designates the location of the fuel-injection and u(x) and uxe2x80x2 (x) designate the flow velocity and, respectively, its instantaneous time change, whereas xcfx84 designates the time delay, which expresses the fact that fuel in homogeneity which occurs at the fuel-injection device is not immediately felt at the flame but only after it has been transported by the average flow from the injection location to the flame front. In a self-igniting combustion apparatus, xcfx84 is determined by the kinematics of the chemical reactions, which determine the location of the flame. In a conventional combustion apparatus with premixing, however, the flame is anchored with a flame holder, which may assume different configurations (bluff body, V-gutter, recirculation zone or the like). In this case, the time delay depends on the average flow velocity and the distance between injection location and flame holder. In each case, the time delay can be described approximately by                     τ        =                              ∫            0            l                    ⁢                                    ⅆ              x                                      U              ⁡                              (                x                )                                                                        (        3        )            
where l designates the distance between the injection location and the flame front, whereas U(x) is the average flow velocity in the premix zone of the burner, with which average flow velocity the fuel in homogeneity in the flow is transported from the injection device to the flame.
In summary, it may be stated that the equation (2) expresses the fact that an instantaneous increase in the velocity of the air flowing past the fuel-injection device (first term on the right-hand side of the equation) leads to a dilution of the fuel/air mixture and to a corresponding reduction in the heat release, whereas a pressure increase at the fuel-injection device (second term on the right-hand side of the equation) reduces the instantaneous fuel mass flow and thus likewise reduces the heat-release rate. Even if the fuel-injection device is acoustically xe2x80x9cstiffxe2x80x9d (i.e. xcex94xe2x86x92∞) - fuel in homogeneity can be produced at the injection device.
As far as the thermoacoustic stability is concerned, a time delay, as occurs in equation (2), generally permits a resonant feedback and an amplification of infinitesimal disturbances. Of course, the exact conditions and frequencies during which self-excited vibrations occur also depend on the average flow conditions, to be precise in particular on the flow velocities and temperatures, and on the acoustics of the combustion apparatus, such as, for example, the boundary conditions, natural frequencies, damping mechanisms, etc. Nonetheless, the relationship between the acoustic properties and the fluctuations in the heat release, as described in equation (2), constitute a serious threat to the thermoacoustic stability of the combustion apparatus. A way of suppressing this mechanism from the very start is therefore desirable.
In principle, it is conceivable within the limits of the above-mentioned considerations to suppress thermoacoustic instabilities by a distribution of different time delays on the time axis. In this case, the injected fuel is split up into two or more individual flows or xe2x80x9clotsxe2x80x9d which all have different time delays and correspondingly different phases with respect to one another. Ideally, such splitting-up into various fuel flows should result in fluctuations in the heat release Qi (i=1, 2, . . . ) in such a way that                                           ∑            i                    ⁢                                    ∫              0              T                        ⁢                                                            Q                  i                                ⁡                                  (                  t                  )                                            ⁢                              ⅆ                t                                                    =        0                            (        4        )            
would apply. This would ensure that the Rayleigh criterion (1) cannot be fulfilled. In practice, such an exact extinction is neither possible nor necessary; it is sufficient to reduce the intensity of the resonant feedback to such an extent that the dissipative effects within the system are greater than the amplification mechanisms.
It has been proposed (DE-A1-198 09 364), for a burner or a plurality of burners working in parallel in a combustion chamber, to inject fuel in an axially graduated manner at different axial distances from the location of the heat release in order to uncouple the fuel from the combustion and reduce the dynamic pressure amplitude of the combustion flame. However, such a solution has the disadvantage that the desired graduated fuel-injection requires complicated equipment to achieve the axial graduation. This is because, if fuel is injected in an axially graduated manner inside a burner, a plurality of separate injection openings arranged one behind the other are necessary. On the other hand, if a plurality of parallel burners having different axial injection locations are used, the burners must be produced individually on account of their different configurations, which makes manufacture and stock-keeping considerably more expensive.
In one aspect of the invention, a combustion apparatus is provided that achieves a distribution of delay times in the injection of fuel without having to change the location of fuel injection.
The various fuel-injection devices are provided with different acoustic impedance or stiffness with respect to the acoustic signal outside the spray devices, resulting in a different phase of the fluctuations in the fuel mass flow. In this case, the quasi-steady assumptions which are expressed by the second term on the right-hand side of equation (2) are no longer appropriate. On the contrary, a detailed description of the acoustic system of the fuel supply is necessary in order to obtain a sufficiently accurate description of the dynamic properties. Nonetheless, the principle is clear: if the fuel-injection device is acoustically sufficiently xe2x80x9csoftxe2x80x9d and the frequency of the excitation, i.e., the pressure signal pxe2x80x2(x1), lies close to the natural frequency of the fuel inlet, a phase displacement develops between the excitation and the response. Of particular interest here is the case where the natural frequency of a fuel-injection device lies above the excitation frequency, and the natural frequency of another fuel-injection device lies below this natural frequency. The fluctuations in the fuel-spraying would be exactly in phase opposition in this case.
In a preferred embodiment of the combustion apparatus according to the invention, the fuel-injection devices each have a predetermined pressure drop of the fuel, and the pressure drop is selected to be different for at least two fuel-injection devices in order to realize the different acoustic impedances of the fuel-injection devices. This embodiment has the advantage that no changes are necessary in the fuel-distribution system located upstream of the fuel-injection devices.
Another preferred embodiment is distinguished by the fact that the fuel-injection devices are each supplied with fuel by a separate fuel-distribution line, and additional means which vary or set the acoustic impedance of the fuel-injection devices are provided in the fuel-distribution lines. This embodiment has the advantage that the spraying devices can remain unchanged, since the requisite changes are made in the fuel-distribution system located upstream. In this case, the additional means for varying the acoustic impedance may comprise, in particular, resonance cavities which are arranged in the fuel-distribution lines. Resonance cavities of the same type can be arranged in all the fuel-distribution lines, with the different acoustic impedance for various fuel-injection devices being achieved by positioning the resonance cavities at different distances from the fuel-injection devices. Alternatively, different acoustic impedance for different fuel-injection devices can be achieved by arranging resonance cavities only in selected fuel-distribution lines. A suitable burner, in particular, is a double-cone burner, as has been developed and successfully used by the applicant, and as described in detail in U.S. Pat. No. 4,932,861, which is herein incorporated by reference.